The invention relates to a puffer circuit-breaker comprising a casing which is intended to be filled with an electric arc extinguishing fluid, a first closure member disposed inside the casing and defining a first thermal expansion chamber, a second closure member disposed within the first closure element and defining a second thermal expansion chamber, a first contact slidably mounted within the casing to slide axially so as to extend through said first and second thermal expansion chambers in order to co-operate electrically with a second contact stationary within the casing and extending axially inside the second thermal expansion chamber.
The invention is particularly applicable to high voltage and to medium voltage circuit-breakers which have a casing filled with a gas of good dielectric properties, in particular SF.sub.6, and which have neither an extinguishing piston nor a magnet.
U.S. Pat. No. 4,259,555 shows such a puffer circuit-breaker comprising two thermal expansion chambers of different volumes allowing a wide range of electrical currents to be interrupted, for example currents in the range 0 to 25000 amps.